In those supports, the problem arises of permitting simplified positioning of the feet, for example, an extension of various telescopic members with relative locking thereof at the desired extension, without also complicating the closing of the feet in order to put the support back when the work has been completed. It is also desired that the telescopic members be provided with arrangements for relative locking that are suitable for increasing the locking action under load while ensuring easy unlocking when the work has been completed. It is also desired that it should be possible to activate the unlocking operation with a single control operation.
These functions are not all made available at the same time by the supports produced in accordance with the prior art.
For example, it is known to produce feet (otherwise known as legs in pluripod supports) having a pair of elements guided telescopically one inside the other (telescopic members), one of which has locking means of the type in the form of a ring of balls. Locking is effected by expansion of the ring of balls as a result of the relative movement between the balls and a frustoconical expander body.
A first disadvantage of this type of foot is encountered when, as is routine, the telescopic members are produced from a light alloy, for example aluminium alloy. In that case, because the action of the expander body increases under load, that is to say, the force with which the balls are pressed against the inner shell of the telescopic member in question increases in proportion to the load weighing on the foot, conditions may arise in which the radial expansion pressure of the ring is such as significantly to damage the inner shell of the telescopic member against which the balls press. Unlocking is effected by operating a rod which acts on a plate which holds the balls against the expander body. The mechanism is such that it is possible to control only one pair of telescopic members, which limits to an unacceptable extent the height of the foot in the extended condition.